1. Field of the Invention
This invention relates to a radiation image information read-out method and apparatus, wherein a technique for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet is employed, and an operation-processed image signal is obtained from the two image signals.
2. Description of the Related Art
In various fields, such as the medical field, techniques for reading out a radiation image of an object, which has been recorded on a stimulable phosphor sheet, to obtain an image signal, performing predetermined image processing on the image signal, and thereafter displaying a visible image, which is reproduced from the processed image signal, on an image display device, such as a cathode ray tube (CRT) display device, or outputting the visible image on film with a printer, such as a laser printer (LP), have heretofore been utilized. As apparatuses for reading out the radiation image, for example, computed radiography (CR) apparatuses, in which digital image processing techniques utilizing computers are employed, have heretofore been utilized.
The CR apparatuses are radiation image recording and read-out apparatuses. With the radiation image recording and read-out apparatuses, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of a stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal with photoelectric read-out means, such as a photomultiplier. Recently, the CR apparatuses are widely used in practice.
As techniques for photoelectrically detecting light is emitted by a stimulable phosphor sheet, the applicant proposed techniques for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet in, for example, U.S. Pat. No. 4,346,295 and Japanese Unexamined Patent Publication No. 8(1996)-116435. With the proposed techniques, two photoelectric read-out means are located respectively on opposite surface sides of the stimulable phosphor sheet. Also, stimulating rays are irradiated to the two surfaces or only to one surface of the stimulable phosphor sheet, and the light, which is emitted by the stimulable phosphor sheet when it is exposed to the stimulating rays, is photoelectrically detected on the opposite surface sides of the stimulable phosphor sheet.
Specifically, with the techniques for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet, a stimulable phosphor sheet is formed by overlaying a stimulable phosphor layer on a surface of a transparent substrate, and a radiation image is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is fitted on a transparent holder, and two photoelectric read-out means are located respectively above and below the holder. In this state, the light, which is emitted from one surface side of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, is detected by the photoelectric read-out means located above the holder, and an image signal (a one surface side image signal) is thereby detected. Also, the light, which is emitted from the other surface side of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, is detected by the photoelectric read-out means located below the holder, and an image signal (an other surface side image signal) is thereby detected.
Thereafter, an addition operation is performed on the image signal components of the two image signals having been detected from the opposite surfaces of the stimulable phosphor sheet (i.e., the one surface side image signal and the other surface side image signal), which image signal components represent corresponding pixels on the one surface and the other surface of the stimulable phosphor sheet, and an addition image signal (a superposition image signal) is thereby obtained. With the addition operation, high frequency noise occurring at random in each of the one surface side image signal and the other surface side image signal can be smoothed. Also, since the emitted light is collected from the two surfaces of the stimulable phosphor sheet, the light collecting efficiency can be enhanced. As a result, a superposition image, which has good image quality with an enhanced signal-to-noise ratio, can be reproduced from the addition image signal.
Also, as techniques for obtaining radiation image information by utilizing stimulable phosphor sheets, or the like, subtraction processing (subtraction operation) techniques have heretofore been known. With the subtraction processing techniques, a plurality of radiation images are recorded under different conditions and are then read out to obtain a plurality of image signals. Thereafter, a subtraction operation is performed on the plurality of the image signals, and a subtraction image signal is thereby obtained. In this manner, a subtraction image corresponding to the difference between the plurality of the radiation images, i.e. a subtraction image in which only a pattern of a specific object part (hereinbelow referred to also as a pattern of a tissue, a structure, or the like) in the radiation images has been enhanced or extracted, is obtained. Basically, the subtraction processing techniques may be classified into a temporal (time difference) subtraction processing technique and an energy subtraction processing technique. The applicant proposed various energy subtraction processing techniques utilizing stimulable phosphor sheets in, for example, U.S. Pat. Nos. 4,855,598 and 4,896,037. In order for the plurality of the image signals for the subtraction processing to be obtained, it is also considered to utilize the aforesaid technique for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet.
Also, in order for a signal-to-noise ratio to be enhanced over the entire frequency band of an image obtained from the addition operation or the subtraction processing, the applicant proposed a technique for performing masking processing on the image signal obtained from each of the opposite surfaces of a stimulable phosphor sheet, such that a parameter (a weight factor) appropriate for each of the opposite surfaces of the stimulable phosphor sheet and each of frequency bands is capable of being utilized, as disclosed in, for example, Japanese Unexamined Patent Publication No. 7(1995)-319092.
When the image signal is to be obtained from each of the opposite surfaces of the stimulable phosphor sheet, read-out conditions (read-out modes) are set, and thereafter the operation for reading out the radiation image information is performed. Examples of the read-out conditions include a level of stimulation energy (i.e., an intensity of the stimulating rays irradiated to the stimulable phosphor sheet), an image signal gathering range (i.e., a range of a radiation dose, over which the image signal is capable of being detected), a read-out pixel density, and a pixel sampling frequency. (The image signal gathering range is also referred to as the read-out gain.) The read-out conditions are set in accordance with the performance required of the image to be obtained. The image signal gathering ranges with respect to the opposite surface sides of the stimulable phosphor sheet are set to be identical with each other.
However, in cases where the level of stimulation energy, the pixel sampling frequency, or the like, varies, the image signal gathering range with respect to each of the opposite surface sides of the stimulable phosphor sheet, the frequency response of the image signal obtained from each of the opposite surfaces of the stimulable phosphor sheet, or the like, varies. Therefore, if the addition operation or the subtraction operation is performed by use of predetermined values of parameters for the operation processing, the problems will occur in that an appropriate image cannot always be obtained.
For example, in cases where the level of stimulation energy alters, the intensity of the light emitted from each of the opposite surfaces of the stimulable phosphor sheet alters. As a result, the frequency response and noise characteristics of the image signal, which is obtained from each of the opposite surfaces of the stimulable phosphor sheet, varies. Therefore, for example, if the image signals, which have been obtained from the opposite surfaces of the stimulable phosphor sheet, are weighted with the identical values of weight factors and subjected to the addition operation, much noise will occur in the operation-processed image, which is obtained from the addition operation, and an appropriate image cannot always be obtained.
Also, the inventors conducted extensive research and found that, besides the intensities of the light emitted from the opposite surfaces of the stimulable phosphor sheet, the ratio between the intensities of the light emitted from the opposite surfaces of the stimulable phosphor sheet also alter in accordance with the level of stimulation energy. It was also found that, in cases where the ratio between read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet is kept at a predetermined value, the image signal gathering range varies for the opposite surface sides of the stimulable phosphor sheet when the level of stimulation energy alters. In other words, if the ratio between the read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet is not adjusted in accordance with the level of stimulation energy, the image signal gathering ranges with respect to the opposite surface sides of the stimulable phosphor sheet cannot be kept identical with each other, and an operation-processed image having good image quality cannot be obtained.
Further, in cases where the read-out pixel density or the pixel sampling frequency varies, if the parameters for the masking processing are kept at predetermined values, the frequency characteristics of the image signal will vary, and therefore it will not always be possible to utilize a parameter (a weight factor) appropriate for each of the opposite surface sides of the stimulable phosphor sheet and each of frequency bands. In such cases, an operation-processed image having good image quality cannot be obtained.
The primary object of the present invention is to provide a radiation image information read-out method, wherein a technique for detecting light emitted from opposite surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the opposite surfaces of the stimulable phosphor sheet is employed, and wherein an appropriate operation-processed image is capable of being obtained reliably regardless of read-out conditions having been set.
Another object of the present invention is to provide an apparatus for carrying out the radiation image information read-out method.
The present invention provides a radiation image information read-out method, comprising the steps of:
i) exposing a stimulable phosphor sheet, on which radiation image information has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation,
ii) detecting light emitted from opposite surfaces of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, two original image signals representing the radiation image information being thereby acquired, and
iii) performing operation processing on the two original image signals and in accordance with predetermined operation processing parameters, an operation-processed image signal, which represents the radiation image information, being thereby obtained,
wherein a ratio between read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet and/or the predetermined operation processing parameters with respect to at least either one surface side of the stimulable phosphor sheet are controlled in accordance with read-out conditions employed at the time of the image signal acquisition.
The present invention also provides an apparatus for carrying out the radiation image information read-out method in accordance with the present invention. Specifically, the present invention also provides a radiation image information read-out apparatus, comprising:
i) stimulating ray irradiation means for irradiating stimulating rays to a stimulable phosphor sheet, on which radiation image information has been stored, the stimulating rays causing the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation,
ii) image signal acquiring means for detecting light emitted from opposite surfaces of the stimulable phosphor sheet when the stimulable phosphor sheet is exposed to the stimulating rays, and thereby acquiring two original image signals representing the radiation image information, and
iii) operation-processed image signal forming means for performing operation processing on the two original image signals and in accordance with predetermined operation processing parameters in order to obtain an operation-processed image signal, which represents the radiation image information,
wherein the improvement comprises the provision of control means for controlling a ratio between read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet and/or the predetermined operation processing parameters with respect to at least either one surface side of the stimulable phosphor sheet, the controlling being performed in accordance with read-out conditions employed at the time of the image signal acquisition.
In the radiation image information read-out method and apparatus in accordance with the present invention, the operation processing, which is performed on the two original image signals and in accordance with the predetermined operation processing parameters, may be typically the addition operation or the subtraction operation described above. However, the operation processing is not limited to the addition operation and the subtraction operation.
Also, the ratio between the read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet may be controlled by controlling the read-out sensitivity with respect to one of the opposite surface sides of the stimulable phosphor sheet. Alternatively, both the read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet may be controlled, and the ratio between the read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet may thereby be controlled.
Further, as a technique for controlling the read-out sensitivity, one of various techniques maybe employed, with which the level of the image signal corresponding to the X-ray dose of a desired read-out range, e.g. the center value of the image signal corresponding to the center point of the desired read-out range, can be altered. As the technique for altering the center value of the image signal corresponding to the center point of the desired read-out range, for example, a technique for altering the voltage (high voltage) applied to a photomultiplier, which photoelectrically detects the emitted light, or a technique proposed by the applicant in, for example, Japanese Unexamined Patent Publication No. 2(1990)-108175, wherein signal transform is performed in accordance with a read-out sensitivity (a S value) in exposure data recognizer (EDR) processing in which no preliminary read-out operation is performed, may be employed.
Ordinarily, when the operation for reading out the radiation image information is performed, the image signal gathering range (i.e., the read-out gain) is set at a predetermined value. As a technique for setting the image signal gathering range, one of various techniques may be employed, with which the width of the image signal corresponding to the desired image signal gathering range is capable of being altered. For example, it is possible to employ a technique, wherein a gain of an amplifier for amplifying the signal obtained from the photomultiplier is altered, or a technique, wherein signal transform is performed in accordance with a latitude (a L value) in the EDR processing in which no preliminary read-out operation is performed.
In order for the ratio between the read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet to be controlled in accordance with the read-out conditions, a table, which represents correspondence relationship between various read-out conditions and read-out sensitivity ratios appropriate for the read-out conditions, may be prepared previously. Also, reference may be made to the table, and the read-out sensitivity ratio corresponding to the read-out conditions, which are employed in the operation for reading out the radiation image information, may thereby be set.
In order for the operation processing parameters to be controlled in accordance with the read-out conditions, a table, which represents correspondence relationship between various read-out conditions and operation processing parameters appropriate for the read-out conditions, may be prepared previously. Also, reference may be made to the table, and the operation processing parameters corresponding to the read-out conditions, which are employed in the operation for reading out the radiation image information, may thereby be set.
As for certain read-out conditions, a read-out mode, in which the detection of an image signal is performed only on one surface side of the stimulable phosphor sheet, may often be set. Also, it may often occur that a read-out mode, in which the detections of image signals are performed on the opposite surface sides of the stimulable phosphor sheet, is set, but a stimulable phosphor sheet, which allows the detection of the image signal from its one surface alone, is utilized. In such cases, the operation processing parameters may be set such that an image is constituted from only the image signal detected from the one surface of the stimulable phosphor sheet.
With the radiation image information read-out method and apparatus in accordance with the present invention, the ratio between the read-out sensitivities with respect to the opposite surface sides of the stimulable phosphor sheet and/or the operation processing parameters with respect to at least either one surface side of the stimulable phosphor sheet are controlled in accordance with the read-out conditions employed at the time of the image signal acquisition. Therefore, in cases where different read-out conditions are set, the operation for reading out the radiation image information is capable of being performed by setting the read-out sensitivity ratio and/or the operation processing parameters appropriate for the read-out conditions having been set, and an appropriate image is capable of being obtained.
Also, with the radiation image information read-out method and apparatus in accordance with the present invention, in cases where the specifications of the stimulable phosphor sheet are such that the stimulable phosphor sheet allows the detection of the image signal from its one surface side alone, the operation processing parameters may be set such that an image is constituted from only the image signal detected from the one surface of the stimulable phosphor sheet. In this manner, the image is capable of being reproduced from only the image signal, which has been detected accurately from one surface side (e.g., the stimulating ray irradiation surface side) of the stimulable phosphor sheet. Specifically, in cases where the read-out conditions concerning the specifications of the stimulable phosphor sheet are set, the image read-out processing is capable of being performed appropriately. Therefore, regardless of the read-out conditions, an appropriate image is capable of being obtained.